Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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A Few Crustaceous
Lichen-Like Variolariae, Leprariae, And Urceorariae, Were Scattered
About Upon The Basalts.
The lavas which are not covered with
volcanic ashes remain for ages without any appearance of
vegetation.
On the African soil excessive heat and lengthened
drought retard the growth of cryptogamous plants.
The basalts of Graciosa are not in columns, but are divided into
strata ten or fifteen inches thick. These strata are inclined at an
angle of 80 degrees to the north-west. The compact basalt
alternates with the strata of porous basalt and marl. The rock does
not contain hornblende, but great crystals of foliated olivine,
which have a triple cleavage.* (* Blaettriger olivin.) This
substance is decomposed with great difficulty. M. Hauy considers it
a variety of the pyroxene. The porous basalt, which passes into
mandelstein, has oblong cavities from two to eight lines in
diameter, lined with chalcedony, enclosing fragments of compact
basalt. I did not remark that these cavities had the same
direction, or that the porous rock lay on compact strata, as
happens in the currents of lava of Etna and Vesuvius. The marl,* (*
Mergel.) which alternates more than a hundred times with the
basalts, is yellowish, friable by decomposition, very coherent in
the inside, and often divided into irregular prisms, analogous to
the basaltic prisms. The sun discolours their surface, as it
whitens several schists, by reviving a hydro-carburetted principle,
which appears to be combined with the earth. The marl of Graciosa
contains a great quantity of chalk, and strongly effervesces with
nitric acid, even on points where it is found in contact with the
basalt. This fact is the more remarkable, as this substance does
not fill the fissures of the rock, but its strata are parallel to
those of the basalt; whence we may conclude that both fossils are
of the same formation, and have a common origin. The phenomenon of
a basaltic rock containing masses of indurated marl split into
small columns, is also found in the Mittelgebirge, in Bohemia.
Visiting those countries in 1792, in company with Mr. Freiesleben,
we even recognized in the marl of the Stiefelberg the imprint of a
plant nearly resembling the Cerastium, or the Alsine. Are these
strata, contained in the trappean mountains, owing to muddy
irruptions, or must we consider them as sediments of water, which
alternate with volcanic deposits? This last hypothesis seems so
much the less admissible, since, from the researches of Sir James
Hall on the influence of pressure in fusions, the existence of
carbonic acid in substances contained in basalt presents nothing
surprising. Several lavas of Vesuvius present similar phenomena. In
Lombardy, between Vicenza and Albano, where the calcareous stone of
the Jura contains great masses of basalt, I have seen the latter
enter into effervescence with the acids wherever it touches the
calcareous rock.
We had not time to reach the summit of a hill very remarkable for
having its base formed of banks of clay under strata of basalt,
like a mountain in Saxony, called the Scheibenbergen Hugel, which
is become celebrated on account of the disputes of volcanean and
neptunean geologists.
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